Embodiments of the invention relate generally to heated tip fuel injectors, and more particularly, to heat transfer in an induction-heated fuel injector.
There is a continued need for improving the emissions quality of internal combustion engines. At the same time, there is pressure to minimize engine crank times and time from key-on to drive-away, while maintaining maximum fuel economy. Those pressures apply to engines fueled with alternative fuels such as ethanol as well as to those fueled with gasoline.
During cold temperature engine start, the conventional spark ignition internal combustion engine is characterized by high hydrocarbon emissions and poor fuel ignition and combustibility. Unless the engine is already at a high temperature after stop and hot-soak, the crank time may be excessive, or the engine may not start at all. At higher speeds and loads, the operating temperature increases and fuel atomization and mixing improve.
During an actual engine cold start, the enrichment necessary to accomplish the start leaves an off-stoichiometric fueling that materializes as high tail-pipe hydrocarbon emissions. The worst emissions are during the first few minutes of engine operation, after which the catalyst and engine approach operating temperature. Regarding ethanol fueled vehicles, as the ethanol percentage fraction of the fuel increases to 100%, the ability to cold start becomes increasingly diminished, leading some manufacturers to include a dual fuel system in which engine start is fueled with conventional gasoline and engine running is fueled with the ethanol grade. Such systems are expensive and redundant.
Another solution to cold start emissions and starting difficulty at low temperature is to pre-heat the fuel to a temperature where the fuel vaporizes quickly, or vaporizes immediately (“flash boils”), when released to manifold or atmospheric pressure. Pre-heating the fuel replicates a hot engine as far as fuel state is considered.
A number of pre-heating methods have been proposed, most of which involve preheating in a fuel injector. Fuel injectors are widely used for metering fuel into the intake manifold or cylinders of automotive engines. Fuel injectors typically comprise a housing containing a volume of pressurized fuel, a fuel inlet portion, a nozzle portion containing a needle valve, and an electromechanical actuator such as an electromagnetic solenoid, a piezoelectric actuator or another mechanism for actuating the needle valve. When the needle valve is actuated, the pressurized fuel sprays out through an orifice in the valve seat and into the engine.
One technique that has been used in preheating fuel is to inductively heat metallic elements comprising the fuel injector with a time-varying magnetic field. Exemplary fuel injectors having induction heating are disclosed in U.S. Pat. No. 7,677,468, U.S. patent application Ser. Nos.: 20070235569, 20070235086, 20070221874, 20070221761 and 20070221747, the contents of which are hereby incorporated by reference herein in their entirety. The energy is converted to heat inside a component suitable in geometry and material to be heated by the hysteretic and eddy-current losses that are induced by the time-varying magnetic field.
The inductive fuel heater is useful not only in solving the above-described problems associated with gasoline systems, but is also useful in pre-heating ethanol grade fuels to accomplish successful starting without a redundant gasoline fuel system.
Once a useful heating method is available, the next challenge is transferring the heat from the appropriate loss component to the fuel to be heated. Conventional methods include convection and conduction heat transfer from the selectively heated metal components to the fuel. These conventional methods suffer from a limit imposed by the thermal conductivity and surface area of the materials involved. If one attempts to increase the heat flux into a given volume of fluid simply by increasing the temperature of the selectively heated components, the result is often exceeding the vapor pressure of the fuel for that new higher temperature and the generation of a wall film of vaporized fuel, the film boiling regime, that then reduces thermal conductivity because it is less efficient to transfer heat into a gas than to transfer heat into a liquid.
Embodiments of the invention provide improved heat transfer, overcome difficulties associated with alternative solutions, and avoid the generation of film boiling.